1. Field of the Invention
The invention generally relates to a lens, and more particularly, to a fixed-focus lens.
2. Description of Related Art
An existing small light emitting diode (LED) projectors usually produces a brightness of merely 100 lumens. To increase the brightness of a small LED projector without increasing the power of LED, a lens with a large aperture has to be adopted to increase the light utilization efficiency.
Image aberration has always been a major problem in the design of lenses with large apertures (for example, with their f-numbers smaller than 2). One technique for resolving this problem is to adopt aspheric lenses. For example, a lens with two aspheric lenses is disclosed in U.S. Pat. No. 5,920,433. However, because this lens is composed of 10 lenses and the total length of the lens is over 75 mm, the overall size of the lens is very large.
In addition, a lens with two aspheric lenses or at least one molded glass lens is disclosed in U.S. Pat. No. 7,397,610. Because the lenses in foregoing lens are grouped into three lens groups and the lens focuses by moving the second lens group, the fabrication cost of the lens is greatly increased. Besides, the f-number of foregoing lens is only between 1.74 and 2.16. Thus, if the aspheric lenses are removed to reduce the fabrication cost and the image aberration is to be reduced, more lenses have to be adopted. For example, in U.S. Pat. No. 7,173,766, 15 lenses are disposed in order to reduce the image aberration. On the other hand, a lens with a first lens group and a second lens group is disclosed in Taiwan Patent No. 201011337, wherein the first lens group and the second lens groups respectively have a positive dioptre. The first lens group and the second lens group both have aspheric lenses. However, foregoing lens has an f-number of 3.24 and a small aperture.